1. Field of the Invention
The present invention relates to a centrifuge rotor shaft assembly, and more particularly to a centrifuge assembly where a diaphragm is disposed about the rotor shaft assembly to permit the rotor shaft assembly to pivot while substantially limiting horizontal displacement thereof. Also, a member situated between a rotor shaft and a rotor shaft substantially limits vertical displacement of the rotor shaft, while allowing angular deflection of the rotor shaft with respect to the drive shaft.
2. Description of the Prior Art
A centrifuge instrument is a device by which liquid samples may be subjected to a centrifugal force. The samples are typically carried in tubes situated within a member known as a centrifuge rotor. The rotor is mounted at the top of a rotor shaft, which is connected to a drive shaft that provides a source of motive energy. Centrifuge drive systems must be designed to accommodate unbalanced rotating loads. The imbalance may exist initially when loading samples into the centrifuge rotor, or it may result from a tube failure during operation of the centrifuge. The imbalance represents a non-uniform distribution of matter throughout the mass of the rotor.
Any given mass, or centrifuge rotor, has a geometric center based on the dimensions of the mass, and a mass center based on the distribution of matter within the mass. The mass center is also referred to as the center of gravity. In an actual mass or centrifuge rotor, the mass center is offset from the geometric center due to machining errors and density variations. A rotating mass mounted on a drive and suspension system, has a critical speed at which the mass laterally shifts its axis of rotation from rotating about its geometric center to rotating about its mass center.
Centrifuge drive systems operate below and above a critical speed. Below the critical speed, the centrifuge rotor rotates about its geometric center. Above the critical speed, the centrifuge rotor attempts to rotate about its mass center. Because centrifuge drive and suspension systems need to have some type of spring in the system to allow the transition through critical speed, the centrifuge rotor approaches rotation about its mass center. A vibration is induced because centrifuge rotor mass center and the centerline of the drive system do not fully align. The amount of vibration that the rotor produces at a given speed is dependent on the distance between the rotor""s mass center and drive geometric center. If the components of the drive system for the centrifuge are rigidly interconnected, then the vibration would subject the drive system to damaging stresses that could possibly destroy the centrifuge. Accordingly, centrifuge drive systems are typically designed to enjoy a certain degree of flexibility.
For a centrifuge rotor to approximate rotation about its mass center, the rotor shaft must be allowed to horizontally shift its axis of rotation. Accordingly, two flexible joints are required between the drive shaft and the rotor shaft. Flexible shafts and gyros, which are well known in the prior art, both allow the required horizontal shift.
A flexible shaft must bend or deflect in order to allow a rotor to spin about its mass center. The greater the flexibility of the shaft, the further it can be deflected to accommodate the horizontal shift and thus reduce the load on the centrifuge motor bearings, motor suspension and instrument frame. However, there is a tradeoff. Greater flexibility is generally achieved by reducing the diameter of the flexible shaft. Smaller diameter shafts have a greater difficulty in making the critical speed transition, and they can be more easily damaged by an unbalanced rotor or by a rotor that has been dropped on the shaft. Smaller diameter shafts also limit the amount of torque that can be transmitted, thus limiting the acceleration rate.
Gyro systems are more robust and less expensive to replace than flexible shaft systems. A gyro system is basically comprised of a rotor shaft pivotally connected to a drive shaft or motor shaft through an intermediate coupling. The intermediate coupling serves as a universal joint that allows the axis of the rotor shaft to assume a position different from that of the drive shaft. The centrifuge rotor is connected to the rotor shaft with a flexible coupling.
The problem associated with centrifuge operation above critical speed is well recognized in the prior art. The following patents illustrate several mechanisms that have been developed to reduce vibrations.
U.S. Pat. No. 3,770,191 (Blum) discloses a centrifuge drive system that automatically causes the center of gravity of a rotor to become aligned with the axial center of the drive system. An articulated rotor shafts permits lateral movement of the rotor whereby the geometric center of the rotor can be displaced so that its center of gravity becomes aligned with the axis of the drive system. A sliding block element is disposed about the articulated rotor shaft to reduce undue vibration of the shaft.
U.S. Pat. No. 4,568,324 (Williams) discloses a drive shaft assembly including a damper disposed between a flexible shaft and a bearing shaft. The damper accommodates the flexure of the flexible shaft while damping vibrations that are imposed on the flexible shaft by a rotor.
U.S. Pat. No. 5,827,168 (Howell) discloses a disk, rotatably attached to a centrifuge drive shaft, for reducing vertical vibrations of the drive shaft. Damping bearings are positioned against a surface of the disk to reduce vibrations thereof.
FIG. 1 shows a cross section of a typical centrifuge gyro drive shaft assembly of the prior art. A gyro housing 10 generally encloses one end of a rotor shaft 15 and one end of a drive shaft 25, which are interconnected through a coupling 20. The other end of drive shaft 25 is housed within a motor 40. Rotor shaft 15 is supported within gyro housing 10 by bearings 30a and 30b, and flexible mounting 35. The flexible mounting 35 is composed of a bearing housing 36 and two elastomeric rings 37a and 37b. A rotor (not shown) is positioned on top of rotor shaft 15.
At rest, and at speeds below the critical speed, rotor shaft 15 and drive shaft 25 share a common vertical axis 45. During centrifuge operation, motor 40 provides a rotational motive force that rotates drive shaft 25, coupling 20 and rotor shaft 15. Motor 40 accelerates, thus increasing the angular velocity of rotor shaft 15. At the critical speed, the rotational axis of rotor shaft 15 shifts both horizontally and at an angle away from vertical axis 45. This shift is permitted by flexible mounting 35.
Bearings 30a and 30b are horizontally displaced by the horizontal displacement or shift of rotor shaft 15. Flexible mounting 35 compresses and expands to accommodate the displacement of bearings 30a and 30b. As with any spring mass system, the elastic stiffness of flexible mounting 35 results in a resonant frequency that is within the normal operating range of most centrifuge systems.
A drive assembly configured as shown in FIG. 1 suffers from several inherent deficiencies. First, the horizontal shift of rotor shaft 15 and bearings 30a and 30b is itself a source of resonant vibration. A resonance is undesirable in a system where an objective is to minimize vibration. Second, to accommodate the shift and provide an adequate degree of torsional flexibility, flexible mounting 35 is typically composed of an elastomer. As rotational velocity increases, the elastomer becomes less flexible, and less responsive to the horizontal shift. Third, the elastomer is not a very good thermal conductor. Consequenty; heat generated by bearings 30a and 30b is not efficiently dissipated, and they are therefore stressed and susceptible to premature fatigue.
Another undesirable degree of freedom can be found in the vertical movement of rotor shaft 15. Because bearings 30a and 30b are mounted by elastomeric rings 37a and 37b, rotor shaft 15 can move vertically. This vertical movement introduces another mode of vibration at a resonant frequency within the normal operating range of most centrifuge systems.
There is a need for a centrifuge drive assembly that can accommodate the tendency of a rotor to shift its axis of rotation from its geometric center to its mass center while minimizing vibration introduced by horizontal displacement of the drive shaft assembly.
There is also a need for a centrifuge drive assembly that minimizes vibration caused by a vertical displacement of a rotor shaft while allowing angular deflection of the rotor shaft with respect to a drive shaft. SUMMARY OF THE INVENTION
The present invention provides a centrifuge assembly that comprises a rotor shaft assembly and a diaphragm disposed about the rotor shaft assembly. The diaphragm permits the rotor shaft assembly to pivot off a vertical axis while horizontal displacement of the drive shaft assembly is substantially limited.
This unique centrifuge assembly typically comprises a rotor, a rotor shaft assembly and a diaphragm flexibly secured about the rotor shaft assembly. The rotor shaft assembly may include a rotor shaft coupled to the drive shaft via an intermediate coupling, and, optionally, a gyro housing enclosing one end of the rotor shaft and one end of the coupling.
In one embodiment, the diaphragm is comprised of a plurality of radially directed bars.
In a second embodiment, the diaphragm is comprised of an inner flange and an outer flange having a common center point. The flanges are connected by radially directed bars.
In a third embodiment, the diaphragm is a disk with a centrally located hole. The disk provides flexible security throughout a 360xc2x0 arc.
The centrifuge may additionally comprise one or more springs to vertically support the rotor shaft assembly. The springs can be situated beneath the base of the rotor shaft assembly, or formed from an elastomeric ring and disposed about a load bearing perimeter of the rotor shaft assembly, or can be incorporated into a drive coupling.
The present invention allows nutation of the rotor about the rotor shaft assembly and limits horizontal displacement of the axis of rotation of the coupling. Accordingly, the vibration associated with the horizontal displacement is substantially reduced due to the avoidance of any resonant frequencies within the operating range of the centrifuge rotor. That is, the greater the horizontal stiffness, the higher the resonant frequency is pushed above the operating range of the centrifuge.
Additionally, a member situated between a rotor shaft and a drive shaft limits vertical movement of the rotor shaft while allowing angular deflection of the rotor shaft with respect to the drive shaft. The member takes up a gap between the rotor shaft and the drive shaft caused by manufacturing tolerances. In one embodiment, the member is comprised of a cylindrical spacer and two disk-shaped pads. In a second embodiment, the member is comprised of a first sleeve disposed substantially around an end of the rotor shaft, a second sleeve disposed substantially around an end of the drive shaft, and a column disposed between the two sleeves.